The influence of the City over the Conservatives has been laid bare by new research showing that more than half of the Tory party's donations since the general election have come from individuals and businesses working in finance.

Hedge funds, financiers and private equity firms contributed more than a quarter of all the Tories' private donations – which this year poured in at a rate equal to £1m a month – the study by the Bureau of Investigative Journalism has found.

The figures show an increase in the proportion of party funds coming from the financial sector, raising fears that the City's financial influence over the Tories is on the rise as key pieces of legislation are discussed by the coalition government.

They come amid growing concerns that some parts of the financial sector, described by Labour leader Ed Miliband this week as "asset strippers" or "predator financiers", are profiting from financial instability.

The senior Labour shadow minister Peter Hain said the figures confirmed that the Tories remain wedded to the few who do well out of the financial and political system. The Liberal Democrats used the research to step up their campaign for changes to party funding.

The Bureau of Investigative Journalism has mapped, for the first time, donations to the Tories from business to the year ending 30 June.

Using analysis from the Electoral Commission and Companies House databases, the researchers found City donations in the 12 months to July accounted for 51.4% of the £12.2m of funds received by Central Office. Hedge funds, financiers and private equity firms contributed £3.3m – 27% – while 50 City donors paid more than £50,000. All donors contributing this amount or more become members of the Leader's Group and qualify for a face-to-face meeting with the prime minister.

The largest contributor across all the business sectors studied by the bureau was hedge funds which donated £1.38m (11.4%). Three of the City's biggest name hedge fund bosses – Michael Farmer, Lord Stanley Fink and Andrew Law – together contributed £636,300. Fink is the party treasurer. The top financier donor was David Rowland, who contributed £1.1m. Rowland has a colourful City career and was forced to resign as party treasurer before he even took up the job because of links to tax havens. He now controls Banque Havilland – which used to be the crashed Icelandic Kaupthing bank business – in Luxembourg and the hedge fund Blackfish Capital Management.

Outside the City, the sector that donated most was industry, including manufacturing and defence. This sector contributed £913,411 (7.5%). A company controlled by Michael Spencer, another former Conservative party treasurer, donated £163,350. He is campaigning against the EU's attempts to introduce a transaction tax on financial trades and threatened on Fridayto shift some of his company's operations from London "extremely rapidly" if the tax was introduced.

Peter Cruddas, the multimillionaire currency trader who grew up on a Hackney housing estate and left school with no qualifications, handed over £123,600, while his business, CMC Markets UK, donated £100,000. He is co-treasurer of the Conservative party, alongside Fink.

But while Spencer and others are now campaigning against potential tax changes, since the coalition came to power several key measures have been introduced that could benefit the Conservative's City backers. Among them is a commitment to reduce corporation tax to 23% by April 2014 and exempting UK resident companies from corporation tax on all profits for their foreign branches.

The figures show the insurance sector has donated £189,400 as the government discusses radical plans to slash the legal aid budget – a measure which critics claim will benefit insurers. Construction companies have donated more than £220,000 amid a lobbying campaign to relax planning rules covering the green belt.

In a separate survey, the Labour MP John Mann disclosed figures that showed that the top three donors – Rowland, Farmer and Fink – had donated almost £10m since 2005. Stuart Wilks-Heeg, executive director of Democratic Audit, said: "What this study tellingly reveals is the scale of the Conservative party's reliance on a variety of City interests at a time when the Conservative-led government is attempting to kick banking reform into the long grass."

Hain said: "The Conservative party has long since been over reliant on donor income from people at the top of the income scale.

"No wonder David Cameron and George Osborne are straining at the leash to cut tax for people earning at least £150,000 a year while asking everyone else to pay the bill for a financial crisis caused by the banks," he said.

The Liberal Democrat peer Lord Oakeshott said: "Big financiers are still the Tories' big backers with hedge fund gamblers and private equity asset strippers leading the way. Labour is being bankrolled by the union bosses. The coalition must act now to clean up party funding."

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Who gave what?

From the celebrity hairstylist to the Oscar-winning screenwriter, Haroon Siddique profiles a dozen top Tory donors

David Rowland, property developer: £1,160,936

Notoriously camera-shy, Rowland was by a distance the Conservatives' largest donor last year. The former tax exile was set to become party treasurer last year but resigned shortly before he was due to start.

Michael Bishop, former airline head: £335,000

Was one of the country's first openly gay senior executives when he headed BMI. Sold stake in airline to Lufthansa for £318m in 2008.